Well, the first bit of advice is to avoid the Image to Video actor. While that would allow you to combine the image with the old, CPU based actors, you get a performance hit any time you try to bring images from the GPU (Core Image) into the CPU (old style Isadora.)

You want to stay within the realm of CI for best performance. In your case, I'd look at the CI Blend With Alpha Mask, which is documented (along with Apple's other CI Plugins) [on this page](https://developer.apple.com/library/mac/documentation/GraphicsImaging/Reference/CoreImageFilterReference/index.html#//apple_ref/doc/filter/ci/CIBlendWithAlphaMask).

Take note that the mask input of CI Blend With Alpha Mask pays attention to the incoming images _alpha_ channel value, not color of the image itself.